Sputtering process for producing single crystal thin films

ABSTRACT

A SPUTTERING PROCESS FOR DEPOSITING THIN, SINGLE CRYSTAL FILMS HAVING BULK PROPERTIES. THE PROCESS IS CHARACTERIZED BY AN EXTREMELY LOW PRE-SPUTTERING TIME, A SUBSTRATE BIAS OF AT LEAST APPROXIMATELY -30 VOLTS, AND LOW DEPOSITION TEMPERATURES. BOTH RF AND DC SPUTTERING ARE USED. IMPROVIDED THIN FILMS HAVING BULK PROPERTIES ARE PRODUCED. SAID THIN FILMS IN PARTICULAR BEING SUPERCONDUCTING MATERIALS, SUCH AS NIOBIUM. THE FILMS HAVE EXCELLENT CHEMICAL PURITY AND LOW DEFECT DENSITY, AND ARE SINGLE CRYSTALS.

April 10, 1973 J. J. cuoMo ET AL 3,726,776

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United States Patent 3,726,776 SPUTTERING PROCESS FOR PRODUCING SINGLE CRYSTAL THIN FILMS Jerome J. Cuomo, Bronx, Ashok F. Mayadas, Somers, and

Robert Rosenberg, Peekskill, N.Y., assignors to International Business Machines Corp., Armonk, N.Y. Filed June 30, 1969, Ser. No. 837,738 Int. Cl. C23c 15/00 U.S. Cl. 204-192 5 Claims ABSTRACT OF THE DISCLOSURE A sputtering process for depositing thin, single crystal films having bulk properties. The process is characterized by an extremely low pre-sputtering time, a substrate bias of at least approximately 30 volts, and low deposition temperatures. Both RF and DC sputtering are used. Improvide thin films having bulk properties are produced, said thin films in particular being superconducting materials, such as niobium. The films have excellent chemical purity and low defect density, and are single crystals.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a sputtering process for producing improved thin films, and more particularly to induced substrate bias sputtering and the single crystal thin films formed thereby, said thin films being in particular superconducting materials.

DESCRIPTION OF THE PRIOR ART Thin films have been formed by a variety of techniques, including both sputtering and evaporation. From studies made of these films, it is apparent that the properties of thin films (those up to approximately 5000 angstroms) are not necessarily the same as the bulk properties. In most cases, it is extremely difficult to produce thin films having properties similar to those of the bulk material. Because thin films are extremely important in applications, such as switching devices, etc., industry is continually studying techniques of producing reproducibly good thin films of various materials.

It is also true that single crystal films have properties which are difierent than those of polycrystalline thin films of the same material. In some cases a polycrystalline thin film can be made in which some of the properties of the polycrystalline film are as good as those of the bulk material itself. However, polycrystalline film has disadvantages with respect to other properties, and the combination of all factors involved may require the use of single crystalline thin films in some applications. Again, much research is being presently conducted to find ways of reproducibly growing single crystal thin films which have properties approaching those of bulk material.

Sputtering is a well known technique for producing thin films. These sputtering methods are either DC sputtering, AC sputtering, or RF sputtering. In DC sputtering, there is a DC potential of approximately 1500-3000 volts applied between the target and the anode, which is grounded. DC sputtering is generally used for conductive materials.

In AC sputtering, a low frequency potential, i.e., approximately 60 c.p.s., is applied between the target and the anode and, consequently, there is a small amount of sputtering from the anode (substrate) in addition to the sputtering which occurs from the target electrode. However, the duty cycle and the applied potential are adjusted so that most of the sputtering is from the target, with the result that there is a net deposition of target atoms on the substrate. An advantage of AC sputtering 3,726,776 Patented Apr. 10, 1973 is that the sputtering can be made asymmetric, so that there is sputtering both from the substrate and from the target. This provides a cleansing of the substrate deposition and leads to more pure deposits.

An RF sputtering system is one in which the aforementioned low frequency source is replaced by a source producing radio frequency voltage waves of about 13.56 megahertz between the target and the anode. RF sputtering is the only way to deposit insulating materials since, by the application of RF potentials, a target insulator can be made negative with respect to a substrate anode, so that a glow discharge can be established therebetween. RF sputtering is normally not used to deposit metals, since these can be easily sputtered by DC sputtering techniques. However, RF sputtering of conductive metals is known, as can be seen by referring to Ser. No. 514,827, which was filed Dec. 20, 1965, and assigned to the present assignee. Also, U.S. 3,347,772 discusses RF sputtering of metals.

It is also known to provide a bias on the substrate of a growing film, in order to obtain cleansing of the growing film during deposition. In this technique, a negative potential is applied to the substrate so that positive gas ions will bombard the growing film, thereby releasing impurities from it. This substrate bias technique has been used in both DC and RF systems and has generally be approximately volts negative substrate bias (IBM Technical Disclosure Bulletin, vol. 10, No. 10, March 1968, p. 1459.

In the formation of thin films of metallic conductors, good single crystal thin films having bulk properties have not been produced. In particular, single crystal thin films of superconductors, such as niobium, molybdenum, niobium nitride, etc., have not been produced having high critical temperature, sharp transition temperatures, high resistivity ratio (the ratio of the resistivity of the material at room temperature to its resistivity at its critical emperature), low defect density, and high chemical purity. For instance, the best thin films of niobium previously produced have shown resistivity ratios approximately 5.7, chemical impurity of 100 parts per rnllion at best, high defect density, and quite low critical temperature.

Further, sputtering techniques are known as methods for producing such metallic conductors; however, DC sputtering is generaly used, and the properties of thin films (1000 A.-10,000 A.) so produced are substantially the same as those produced by other techniques, such as evaporation.

Accordingly, it is a primary object of this invention to provide an improved method for producing thin films of single crystal metallic conductors.

Another object is to provide a method of producing thin films of single crystal metallic conductors having bulk properties.

Still another object of this invention is to provide a method for epitaxially growing thin films of single crystal metallic conductors having bulk properties.

Another object of this invention is to provide a method for growing thin films of single crystal metallic conductors which are very pure throughout their growth cycle.

It is still another object of this invention to provide an improved method of growing single crystal thin films, which method is more efiicient than previously known methods and which provides better thin films.

A still further object of this invention is to provide a method of growing thin film single crystal metallic conductors, in which lower temperatures of deposition are sufficient.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawmgs.

BRIEF SUMMARY OF THE INVENTION The improved method of growing thin films of single crystal metallic conductors, which thin films have properties approximating those of bulk material, is an RF sputtering process. The process includes the application of an induced substrate bias of at least approximately 30 volts throughout film growth. That is, the 'bias on the substrate exists at the time the first few atoms of metallic conductor arrive at the substrate, and such bias is present throughout the film growth. The magnitude of the required substrate bias is that which is sufiicient to attract positive gas ions of sufficient energy to break the bond existing between atoms of the metallic conductor desired to be deposited and impurity atoms which also nucleate on the substrate.

The sputtering of such improved thin films occurs at a substrate temperature of approximately ZOO-650 C., which is lower than that previously reported in the deposition of metallic conductors. In addition, a very small pre-sputtering time is required, of the order of approximately -30 minutes, in sharp contrast with previously suggested pre-sputtering times which are measured in hours. Consequently, this improved method of producing improved single crystal thin films of metallic conductors is comprised of RF sputtering, in which a substrate bias of at least -30 volts exists from the beginning of film growth, the substrate temperature is approximately 200650 C., and the pre-sputtering time is approximately 5 minutes- 30 minutes.

The thin films produced by the above-described process are single crystal, and have thicknesses up to 30,000 angstroms, resistivity ratios greater than 7 (approximately -60), and sharpness of transition temperature of less than 01 K. The films also have a purity of less than 10 parts per million (99.99%) very low defect density, and bulk value critical temperature (9-10 K.).

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an apparatus used to provide improved thin film single crystal metallic conductors.

FIG. 2A is a schematic illustration of a DC sputtering apparatus in which there is DC substrate bias, and both electrically contacted and isolated substrates.

FIG. 2B is a schematic illustration of the growth of metallic conductor atoms on a substrate in electrical contact with the anode of the apparatus illustrated in FIG. 2A.

FIG. 2C is a schematic illustration of the growth of metallic conductor atoms on a substrate which is electrically isolated from the anode of the apparatus shown in FIG. 2A.

FIG. 3A is an illustration of an RF sputtering apparatus in which there is induced RF substrate bias, having substrates which are in electrical contact with the cathode and other substrates which are isolated from said cathode.

FIG. 3B illustrates growth of metallic conductor atoms on a substrate in electrical contact with the cathode of the RF sputtering apparatus shown in FIG. 3A.

FIG. 3C illustrates growth of metallic conductor atoms on a substrate which is electrically isolated from the cathode of FIG. 3A.

FIG. 4 is a graphical plot of resistivity versus substrate bias voltage for thin films of single crystal metallic conductors formed by the disclosed process.

FIG. 5 is a graphical plot of substrate bias voltage versus thickness for thin films of metallic conductor formed according to the disclosed process.

FIG. 6 is a plot of resistivity versus thickness for thin films of metallic conductors formed according to the disclosed process.

FIG. 7 is a plot of resistivity versus substrate temperature, for thin films of metallic conductors formed by the disclosed process.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an RF sputtering system which is used to provide single crystal, thin films of metallic conductors By the use of the disclosed process, an induced substrate bias of at least 30 volts will exist prior to the start of deposition growth. Since the purity of the deposited film depends on the first few hundred angstroms of growth, it is very important that the induced substrate bias exists from the beginning of the growth cycle. By the subject process, an induced bias exists throughout film growth, regardless of whether or not there is electrical continuity of the film across the substrate. The films produced by this method have almost bulk properties, regardless of their thickness.

In FIG. 1, an RF source 10 provide RF voltage waves between the target 12 and the substrate 14. The RF source 10 is coupled to the target 12 via lead 16 and capacitor 18, while the substrate 14 is grounded. The target is attached to electrode 20, which is cooled by a flow of water through tube 21. The substrate is maintained at an elevated temperature of ZOO-650 C. by heater units 22. Cooling coils 24 surround the anode electrode 26.

Helmholtz coils 28 provide a magnetic field of approximately 30-80 gauss, which magnetic field is perpendicular to the target 12 and anode plates 26. The purpose of this magnetic field is to increase the concentration of electrons in the sputtering environment so that the efliciency of sputtering will be increased. The magnetic field also increases the induced bias on the substrate.

In the usual case, extremely high purity films of metallic conductors are produced by establishing the following sputtering conditions:

(1) A very high purity target;

(2) Low initial system pressure of approximately 2X10- torr;

(3) Liquid nitrogen and titanium pump connected to the sputtering system;

(4) A pre-sputter time of 5-30 minutes, at '20 microns argon pressure;

(5) A substrate temperature of ZOO-650 C.;

(6) A power of 200-800 watts on an approximately 4 /2 inch diameter target;

(7) -An applied magnetic field of 30-80 gauss;

(8) Sputtering pressure of approximately 10 microns argon;

(9) A deposition rate of approximately 7-21 angstroms per second.

A metal chamber 30 encloses the previously described apparatus. The chamber has openings (32, 34) in it, one of which is for admitting the ionizable gas, and the other is for creating a vacuum within chamber 30. Of course, electrical power is applied to Helmholtz coils 28 by a lead which also extends from within the chamber. In this case, the ionizable gas is chosen to be argon, although others could be used. A shield 36 surrounds the target. This prevents sputtering from locations other than the actual target source 12 which is to be deposited as a film upon substrate 14. Shutter 38 is movably posi tioned between target 12 and substrate 14. Turning knob assembly 40 is for rotating the substrate from beneath the target 12.

Titanium filament 42 is used to getter active gases, such as oxygen, nitrogen, etc. from within chamber 30 onto cryogenically cooled walls 43 before sputtering begins.

The chamber is evacuated by a vacuum pump to a pressure of approximately 2 10 torr. Titanium filament 42 is used to getter oxygen from the system and then there is pre-sputtering in argon onto shutter 38. This pre-sputtering insures that the surface of target 12 will be clean when actual deposition onto substrate 14 begins. The pre-sputtering time is only approximately -30 minutes and usually less than about 15 minutes, in sharp contrast with usual pre-sputtering times which are measured in hours.

After the pre-sputtering, the shutter 38 is moved away from its position between the target 12 and the substrate 14. The substrate temperature is approximately ZOO-650 0., although the preferred temperature for many materials is approximately 400-450 C. When an RF potential is applied between the target and the substrate, an induced substrate bias, greater than the wall potential, will exist prior to the deposition of target atoms onto the substrate. The RF potential is approximately 1000 volts, although this is not extremely critical. The geometry of the system, the capacitance to ground of the substrate, and the peak-to-peak voltage of the applied RF wave determine the magnitude of the induced substrate bias.

In order to deposit improved thin films, the induced substrate bias must be at least approximately 35 volts in magnitude. That is, an induced substrate bias which is approximately equivalent to the wall potential is not sufiicient to provide good single crystal thin films having bulk properties.

Of course, some of the sputtering conditions mentioned above can be varied while still providing extremely high quality single crystal thin films of metallic conductors. However, it is important to realize that induced substrate bias must exist prior to adhesion of the first few atoms of target material on the substrate in order to obtain good films. Also, the substrate temperature is extremely low for sputtering operations; that is, these temeperatures are in the range of approximately ZOO-650 C. Further, the required presputtering time is extremely small, of the order of 5-30 minutes. Whereas sputtering normally does not lead to single crystal films, the combination of the steps discussed above provides extremely high quality, single crystal thin films having near bulk properties, regardless of the thickness of the film. As the thickness of the thin film decreases, the quality of the films produced by this method contrasts more sharply with those previously reported. The films produced by the disclosed method maintain their high degree of purity even down to thicknesses of a few tens of angstroms.

FIGS. 2A-2C and 3A-3C illustrate the effect of the induced substrate bias. Referring to FIG. 2A, a DC bias sputtering system is shown in which a potential of 2500 volts is applied to the cathode. A bias potential of ---100 volts is applied to the anode. Located on the anode are two substrates 50, 52, one of which (50) is electrically in contact with the anode while the other of which (52) is electrically isolated from the anode by insulator 54. Consequently, one substrate (50) receives the -100 volt substrate bias while the other substrate (52) does not.

FIGS. 2B and 2C illustrate the growth of a sputtered film onto a contacted substrate and onto an isolated substrate 52. In FIG. 2B, the substrate 55 is in electrical contact with the anode 57, which is held at -100 volts. Conductor 51 insures good electrical contact. The small circles 56 on the substrate represent the target atoms which begin to nucleate on the substrate during deposition. In the initial stages of deposition, nucleation occurs at various places on the substrate. There is no electrical continuity across the growing film, since the growth sites are not yet sufiicient to insure that atoms are present throughout the width of the growing film. In fact, studies have shown that electrical continuity across the growing film does not occur until the film reaches a thickness of approximately 100il50 angstroms. Consequently, in FIG. 2113, there is no bias on the growing film prior to the time deposition begins and throughout the beginning of deposition. It is only after approximately 150 angstroms of film are grown that the film is at O volts. The growing film is represented by dashed line 53.

In the plot (FIG. 2B) of thickness versus applied substrate bias, this is apparent. The film is discontinuous until the substrate bias reaches a higher potential. When growth begins, the film is at the wall potential (approximately 6-l0 volts), which is insutficient to support growth of good quality single crystal films. Substrate bias greater than the nominal wall potential cannot be achieved during DC bias sputtering until the growing film becomes electrically continuous across its width. Thus, the initial layer (whose thickness depends on temperature and species) is deposited under a bias which is equivalent to the wall potential. Since impurity trapping, nucleation density, and film orientation depend upon the film bias potential, this film will not be chemically and physically pure until approximately 150 angstroms are deposited. Therefore, various properties of the film such as resistivity and crystallographic perfection, cannot be controlled.

In FIG. 20, the growing film, represented by dashed line 58, is electrically isolated from the anode bias. Insulator 60 separates substrate 62 from anode 64. Here, the only potential is that of the wall potential, which is too small to provide good films through the first 150 angstroms of growth. The entire thickness of film 58 will be grown at the wall potential, as shown in the plot of thickness versus applied substrate voltage.

Considering the first few atoms 59 which deposit on the substrate 62, a group of such atoms have an aifinity for one another and combine together at various growth sites. Impurity atoms on the substrate are also possible growth sites. Chemical bonds are formed between the target atoms which reach the substrate and the impurities which are on the substrate. At substrate potentials which are only approximately that of the wall potential, ionizable gas atoms (such as argon) are not of sufiicient energy to break the afiinity between the desired target atoms and the impurity atoms at the growth sites. Under these conditions, an argon atom will not dislodge an impurity atom from the substrate. However, if the induced bias on the substrate is at least approximately 30 volts, argon ions of sufiicient energy will impinge upon the growing film and will break the bond which exists between the impurity atoms and the desired metal atoms. Also, impurity atoms on the substrate will be removed thereby preventing growth sites around these atoms.

FIG. 3A shows a system in which an RF potential is applied to the cathode, while the anode is grounded. Again, there is a substrate 70 which is in electrical contact with the anode and a substrate 72 which is electrically isolated from the anode by insulator 74. This system differs from that of FIG. 2A in that RF bias sputtering is used here, rather than DC bias sputtering.

FIG. 3B shows the growth of target atoms 76 on a substrate 78 which is in electrical contact with the anode 80. Assuming that the peak-to-peak voltage of the applied RF wave is sutficient and that the geometry and capacity of the system are suitably chosen, a sufliciently large RF bias will be induced on the substrate 78. This RF bias is greater than the normal wall potential and exists throughout film growth, regardless of whether or not there is electrical continuity across the growing film (represented by a dashed line 82). The plot of film thick ness versus substrate bias (FIG. 3B) shows that the RF bias exists throughout the film growth. Because this bias is at least approximately 30 volts, high purity films are grown from the outset.

FIG. 30 illustrates the case where target atoms are grown on a substrate 92 which is electrically isolated from the anode 94 by insulator 96. Because it is an RF applied voltage, there will be an induced RF bias on the substrate regardless of electrical isolation and regardless of film discontinuity in the first few hundred angstroms. The growing film is represented by dotted line 91. Again, the plot of thickness versus induced RF bias shows that the bias is at least 30* volts and exists throughout the film growth.

The fact that there is cleansing of the deposit throughout film growth by sufficiently energetic argon ions allows the use of lower substrate temperatures than are normally used. Also, the pre-sputtering time can be reduced to a time period significantly smaller than that previously recorded. Even with a short pre-sputtering time and low substrate temperatures, extremely high quality films are produced, and these films are single crystals.

Referring to FIG. 4, the resistivity of the deposited thin film is plotted against the DC substrate bias voltage. In this case, the deposited films are niobium deposited on gallium-backed sapphire wafers at 450 C. substrate temperature. In this case, the film thickness was high enough to mask impurities trapped in the first 100 angstroms (where discontinuity takes place), and bulk resistivity of approximately 14.2 microhm-cm. resulted above 30-40 volt biases. In the case of electrically isolated samples run simultaneously, the resistivity was rela tively independent of applied bias and was about 110 microhm-cm. for a 10,000 angstrom film.

In FIG. 5, DC substrate bias voltage is plotted against the thickness of the growing film. Again, this is the case of niobium deposited on a gallium-backed sapphire wafer at 450 C. substrate temperature. The top curve is the case of a sample in electrical contact with the substrate While the bottom curve corresponds to an electrically isolated sample. From these curves, it is apparent that samples which are in electrical isolation increase in thickness at a rate greater than those which are in electrical contact with the substrate. This is to be expected, since with applied substrate bias there is some cleansing of the growing film. It is to be noted that FIGS. 4 and 5 deal with the case of DC bias sputtering. These curves illustrate the measured effects of substrate bias on the growing film.

In FIG. 5, the curve for a sample grown with substrate bias shows a resistance that is essentially that of a bulk sample. Also, there is an overall increase in sputtering eificiency which is due to the applied bias potential.

FIGS. 6 and 7 relate to RF sputtering and in particular RF supttering of single crystal thin films of niobium. During RF sputtering, the wall potential is considerably higher (approximately 50-100 volts). Consequently, biases are obtained on the substrate surfaces prior to the deposition of metal films. Film resistivities equivalent to those of thick films deposited by DC bias are obtained and there is no difierence between films formed on electrically contacted substrates and electrically isolated substrates. In FIG. 6, the resistivity of the film approaches bulk value in films thicker than about 500 angstroms at a substrate temperature of 450 C. Below this thickness, increased resistivity results from size effects (surface and grain boundary scattering) and incomplete coverage of the substrate.

FIG. 7 shows the resistivity of electrically contacted and electrically isolated films to be equivalent over all substrate temperatures used. Of course, these findings strongly indicate that RF sputtering is a desirable technique where metals are to be deposited on isolated substrate surfaces.

In FIG. 7 the topmost curves (A, B) are plots in which the substrate has a gallium backing. Curve A is one in which the substrate is isolated, while curve B represents resistivity vs. temperature when the substrate is metallized, insuring good electrical contact with the anode. Curves (C, D) illustrate the corresponding situations when the deposit is formed on a substrate without the gallium backing.

The process described above is very useful for contacting active device areas, as for example when metals or conductive compounds are to be placed onto insulated substrates, such as is the case when contacting via holes. By using this RF sputtering technique, an immediate bias is achieved on the substrate. The bias is independent of the thickness of the growing metal film, which is not the case for DC bias sputtering. Consequently, increased purity at the initially sputtered critical interface area is obtained and good electrical contacts will be achieved. This is a decited advantage in the manufacture of many semiconductor products, and particularly the fabrication of integrated circuitry.

DEPOSITED FILMS As previously mentioned, metallic conductors deposited by the above described process are single crystals, the thin films of which have properties not found in previously produced films of metallic conductors. In particular, the superconducting materials formed have extremely high resistivity ratio (ratio of resistivity at room temperature to that at the superconductive transition temperature), low resistivities, high critical temperatures, and sharp transition widths. The chemical purity of these films is 310 parts per million, which low defect density, indicating high structural perfection. The films are actually more pure than most reported bulk material, since the resistivity ratio is limited by surface effects in thin films.

EXAMPLE 1 Niobium was deposited on c-axis sapphire (UL-A1203) and grew epitaxially thereon to form a single crystal thin film. The niobium film had a 111 orientation normal approximately to 0001 and was 7000 A. Its properties were:

Room temperature resistivity: 14.2 microhm-cm. Resistivity ratio: 15-50.

Critical temperature: 9.2 K.

Transition temperature width: 0. 1 K. Chemical purity: 10 parts per million.

The sputtering conditions were:

low initial system pressure of 2X10 torr;

liquid nitrogen and titanium pump operating;

pre-sputtering for 5-10 minutes at 20p. argon pressure;

substrate temperature 200-450 C.;

300- watts power on a 4 /2 inch target;

sputtering from target to substrate at approximately 10p.

argon pressure;

deposition rate 7 A. per second.

EXAMPLE 2 Niobium was deposited on polished l00 MgO, and grew epitaxially, having a l00 orientation. The sputtering conditions were essentially the same as those described in Example 1. Electrically, the properties of these Nb films were the same as the properties of bulk Nb. (T,,=9.2, transition width 501 K., chemical purity 510 parts per million, resistivity ratio=1S-60.)

EXAMPLE 3 Niobium was deposited on a-axis A1 0 The Nb grew epitaxially as a single crystal. The orientation of the single crystal Nb was 1l0 in this example. Room temperature resistivity was 14.2 microhm-cm. Again, the sputtering conditions were essentially the same as those described in Example 1, and the film properties were also the same as previously stated.

EXAMPLE 4 Niobium titanium (NbTi) was deposited on sapphire substrates using the disclosed process. One such single crystal thin film had a critical temperature of 9.55:0.05 K. The composition of this film was identical to that of the source material (Nb Ti Bulk values were obtained.

The sputtering conditions were:

an initial system pressure of -2 10 mm;

operation of liquid nitrogen and titanium pumps; pre-sputtering for 15-30 minutes at 10 argon pressures; substrate temperature of approximately 450 C.;

input power of 300 watts on a 4 inch target;

an applied magnetic field of approximately 60 gauss; a deposition rate of approximately A. per second.

EXAMPLE 5 Molybdenum (Mo) has been sputtered onto tat-A1 03, substrates with an induced substrate bias of approximately 100 volts. The films were single crystal having critical temperature =09 K., room temperature resistivity of 7 microhm-cm. Nearly bulk value films were obtained. M0 is a material having properties similar to aluminum and is also useful in semiconductor device metallurgy.

EXAMPLE 6 Aluminum is a useful material for many applications, such as semiconductor contact leads. It was produced in a thin film (100,000 A.), single crystal form by the subject RF sputtering method, with 200 C. substrate temperature. The improved aluminum has a chemical purity between 99.99 and 99.999 and high crystallographic perfection. Its resistivity ratio was greater than 50. Single crystal films were sputtered onto MgO, mica, and a-Al O From the foregoing, it is apparent that many metallic conductors can be produced as thin films by the disclosed novel sputtering method, and that such films are single crystals having nearly bulk properties. The subject method is particularly suitable for producing films capable of superconductivity. The properties of all metallic films so produced are significantly better than that presently reported, and in many cases there is improvement by orders of magnitude. Further, large size (one inch diameter) single crystal films are produced by this method.

What is claimed is:

1. A method of producing single crystal thin films of metallic conductors having a melting temperature greater than about 200 C., said method comprising:

positioning a target of the metallic conductor to be sputtered in a closed chamber and substantially parallel to a support substrate,

evacuating said chamber to a pressure of about 2 X torr,

10 cleansing said target by presputtering in an inert gas atmosphere from the surface of said target to a shutter which is movably positioned between said target and said substrate, said presputtering being for approximately 5-30 minutes, said inert gas atmosphere having a pressure of approximately 20 microns during said presputtering, sputtering materials from said cleansed target to said substrate at a rate of about 721 angstroms per second in an inert gas atmosphere having a pressure of about 10 microns by applying an RF waveform between said target and said substrate of sufficient power to induce an RF bias on said substrate at a time prior to said sputtering step and continuing during said sputtering step, said induced RF bias being a negative bias having a magnitude between about 30 and volts, said substrate having a temperature of approximately 200650 C. during said sputtering.

2. The method of claim 1, wherein said metallic conductor is a superconducting material.

3. The method of claim 2, where said superconducting material is comprised of Nb.

4. The method of claim 1, where said metallic conductor is selected from the group consisting of Mo, Al, and NbTi alloy.

5. The method of claim 1, further including the step of applying a magnetic field of about 30-80 gauss in a direction substantially normal to the substrate during said sputtering.

References Cited UNITED STATES PATENTS 3,021,271 2/ 1962 Wehner 204-192 3,419,761 12/1968 Pennebaker 204-192 3,432,416 3/ 1969 Rairden et a1. 204-192 3,461,054 8/1969 Uratny 204-192 3,471,396 10/ 19 69 Davidse 204-192 JOHN H. MACK, Primary Examiner S. S. KANTE'R, Assistant Examiner 

